A method and an apparatus for tuning a laser includes creating a laser with a path between a cavity end element and a tuning element of an external cavity, both being a high reflective or semitransparent mirror, selecting at least one longitudinal mode of the laser by introducing a dispersion element in the path of the laser, rotating the tuning element about a pivot axis theoretically defined by the intersection of the surface planes of the cavity end element, the dispersion element and the tuning element to tune the laser. The method and apparatus also include provisions for moving the dispersion element along such a predetermined path to at least partly compensate a shift between the real position of the pivot axis and the theoretically defined position.
|
1. A method of tuning a laser, comprising:
providing a laser beam to a path between a cavity end element and a tuning element of an external cavity; selecting a mode of said laser by introducing a dispersion element in said path; rotating said tuning element about a pivot axis to tune said laser, wherein said pivot axis is defined by an intersection of a surface plane of said cavity end element, a surface plane of said dispersion element, and a surface plane of said tuning element; and moving said dispersion element to at least partly compensate a shift between a real position of said pivot axis and said intersection, wherein said moving of said dispersion element is executed corresponding with said rotating of said tuning element.
10. An apparatus for tuning a laser, comprising:
an external cavity for receiving a laser beam in a path between a cavity end element and a tuning element of said external cavity; a dispersion element being introduced in said path for selecting a mode of said laser; a pivot axis being defined by an intersection of a surface plane of said cavity end element, a surface plane of said dispersion element, and a surface plane of said tuning element, about which pivot axis said tuning element can be rotated to tune said laser; and a moving element for moving said dispersion element to at least partly compensate a shift between a real position of said pivot axis and said intersection, wherein said moving element moves said dispersion element corresponding with a rotation of said tuning element.
9. A software program or product, stored on a data carrier, for executing a method of tuning a laser when run on a data processing system, said method comprising:
providing a laser beam to a path between a cavity end element and a tuning element of an external cavity; selecting a mode of said laser by introducing a dispersion element in said path; rotating said tuning element about a pivot axis to tune said laser, wherein said pivot axis is defined by an intersection of a a surface plane of said cavity end element, a surface plane of said dispersion element, and a surface plane of said tuning element; and moving said dispersion element in order to at least partly compensate a shift between a real position of said pivot axis and said intersection, wherein said moving said the dispersion element is executed corresponding with said rotating of said tuning element.
2. The method of
3. The method of
5. The method of
6. The method of
at least approximately evaluating a function that determines said predetermined rotating angle of said dispersion element for generating mode or wavelength hop free rotating of said tuning element within a predetermined tuning range of said tuning element per rotation angle of said tuning element, by: substantially detecting mode or wavelength hops during said rotating of said tuning element and; rotating said tuning element about a predetermined angle until at least one mode or wavelength hop substantially has occurred; rotating said dispersion element about an arbitrary angle; rotating back said tuning element about the predetermined angle; repeating substantially detecting mode or wavelength hops, rotating said tuning element about a predetermined angle, rotating said dispersion element, and rotating back said tuning element about said predetermined angle, while increasing or decreasing said arbitrary angle until substantially no mode or wavelength hops are detected while rotating said tuning element about said predetermined angle, and using an overall arbitrary angle per predetermined angle to evaluate an approximation of said function that determines said rotating angle of said dispersion element per rotating angle of said tuning element.
7. The method of
8. The method of
measuring a rotating angle of a rotating of said dispersion element, comparing said measured value of said rotating angle with a predetermined value, and adjusting said rotating angle when detecting a difference between said measured value and said predetermined value.
11. The apparatus of
12. The apparatus of
13. The apparatus of
14. The apparatus of
15. The apparatus of
17. The apparatus of
18. The apparatus of
a measuring device for measuring a rotating angle of a rotating of said dispersion element, a comparator connected with said measuring device for comparing a measured value of said rotating angle with a predetermined value of said rotating angle, and a controller connected with said moving element for adjusting said rotating angle when said comparator has detected a difference between said measured value and said predetermined value.
|
|||||||||||||||||||||||||
The present invention relates to laser tuning.
In the optical communication industry there is a need for testing optical components and amplifiers with lasers that can be tuned in wavelength continuously without mode hopping. To perform these tests Littman cavities can be used as external cavities to allow single-mode tuning of the laser. The geometry of these cavities is known, see e.g.: Liu and Littman, "Novel geometry for single-mode scanning of tunable lasers", Optical Society of America, 1981, which article is incorporated herein by reference. The advantage of the Littman cavity is that it is possible to tune the wavelength and the optical length of the cavity at the same time by changing only one parameter of the geometry, i.e. the tuning element.
Examples of tunable lasers, in particular based on the Littman geometry, can be found e.g. in U.S. Pat. No. 5,867,512, DE-A-19509922, Wenz H. et al: "Continuously Tunable Diode Laser" in `Laser und Optoelekronik` (Fachverlag GmbH, Stuttgart, DE, Vol.28 No.1, p.58-62, Feb. 1, 1996, XP000775842, ISSN: 0722-9003), Wandt D. et al: "Continuously Tunable External-Cavity Diode Laser with a Double-Grating Arrangement" (Optics Letter, Optical Society of America, Washington, US, vol.22, no.6, Mar. 15, 1997, pages 390-392, XP000690335, ISSN: 0146-9592), DE-A19832750, EP-A-938171, JP-A-05 267768, or U.S. Pat. No. 5,319,668.
The Littman geometry, however, is extremely sensible to deviations of the real geometry with respect to the perfect Littman configuration. This does impose severe requirements on the rotation mount for the tuning element of the Littman cavity. Smallest errors in the positioning of the pivot axis of the tuning element reduce the mode hop free tuning range of the cavity heavily. This requires costly precision when manufacturing and maintaining such tunable lasers.
Therefore, it is an object of the invention to provide improved tuning of a laser. The object is solved by the independent claims.
An advantage of the present invention is the provision of a tunable laser that autonomously and easily compensates for deviations, e.g. a shift of the real position of the pivot axis of the tuning element with respect to the theoretical perfect position of the pivot axis. This compensation is sufficient to provide continuous single-mode tuning within a predetermined tuning range of the tuning element. The compensation is done by moving the dispersion element, preferably along a predetermined path, corresponding with the rotation of the tuning element. Therefore, method and an apparatus of the present invention for tuning of lasers avoid the aforementioned problems of the prior art and provide a tunable laser with a wide mode hop free tuning range without heavy duties to the precision when manufacturing and maintaining such laser.
In a preferred embodiment of the present invention the moving of the dispersion element is done simultaneously with the rotation of the tuning element. This achieves an online correction, so that always the correct position of the dispersion element for the full feedback of the tuning element is guaranteed.
In another preferred embodiment of the invention the correction is done by moving the dispersion element by rotating it by a predetermined rotating angle about a predetermined rotating axis. This way of correction can be easily implemented in the inventive apparatus, e.g. by using a piezo-electric driven rotation stage that can precisely move the respective tuning element of the laser.
In yet another preferred embodiment of the invention the correction is done by a dispersion element which is a diffraction grating and in which the rotating axis is at least not perpendicular, more preferred parallel, to the axes of the rules of the grating. This positioning serves for maximum efficiency of the inventive method and apparatus.
In another preferred example of the invention the method further comprises steps for at least approximately evaluating a function which determines the rotating angle of the dispersion element for generating mode or wavelength hop free rotating of the tuning element within a predetermined tuning range of the tuning element per rotation angle of the tuning element. This evaluation is done by the following steps: step a: substantially detecting mode or wavelength hops during rotation of the tuning element, step b: rotating the tuning element about a predetermined angle until at least one mode or wavelength hop substantially has occurred, step c: rotating the dispersion element about an arbitrary angle, step d: rotating back the tuning element about the predetermined angle of step a, and repeating steps a to d with increasing or decreasing rotating angle of step c until substantially no mode or wavelength hops during rotation of the tuning element are detected in step b, and using the overall rotating angle of step c per rotating angle of step b to evaluate an approximation of the function which determines the rotating angle of the dispersion element per rotating angle of the tuning element. This can be done fast and easy so that a quick adjustment of the apparatus for full feedback of the tuning element is achieved.
After performing the above described determination it is preferred to move the dispersion element according to the approximation function before or while rotating the tuning element.
Other preferred embodiments are shown by the dependent claims.
It is clear that the invention can be partly embodied or supported by one or more suitable software programs, which can be stored on or otherwise provided by any kind of data carrier, and which might be executed in or by any suitable data processing unit.
Other objects and many of the attendant advantages of the present invention will be readily appreciated and become better understood by reference to the following detailed description when considering in connection with the accompanied drawings. The components in the drawings are not necessarily to scale, emphasis instead being placed upon clearly illustrating the principles of the present invention. Features that are substantially or functionally equal or similar will be referred to with the same reference sign(s).
Referring now in greater detail to the drawings,
The tuning element 8 can be rotated by an actuator (not shown) according to arrow 12 about a pivot axis 14 to tune the laser. The pivot axis 14 is theoretically defined by the intersection of the surface plane (indicated by line 6a) of the cavity end element 6, the surface plane (indicated by line 10b) of the dispersion element 10 and the surface plane (indicated by line 8a) of the tuning element 8.
The dispersion element 10 is mounted on one end of an electrically driven bimorph type piezo-electric element (not shown) which serves as the moving element of the invention. One end of the bimorph type piezo-electric element is freely slewable whereas the other end of the bimorph type piezo-electric element is fixed relative to the cavity 2.
The bimorph type piezo-electric element allows moving the dispersion element 10 corresponding, preferably simultaneously with the rotation of the tuning element 8 to compensate a shift between the real position of the pivot axis 14 and the theoretically defined position. This is done preferably by moving the dispersion element 10 along such a predetermined path that the compensation is sufficient to provide continuous single-mode tuning within a predetermined tuning range of the tuning element 8. Moving the dispersion element 10 in this embodiment means rotating the dispersion element 10 according to arrow 16 by a predetermined rotation angle (for the predetermination of the rotation angle see below) about the rotating axis 10a which is substantially parallel to the rules 11 a and substantially lies in the plane of the grating 11.
The apparatus of
For the above mentioned predetermination of the predetermined value of the rotating angle 16 of the dispersion element 10 the following steps can be performed: step a: substantially detecting mode or wavelength hops during rotation of the tuning element 8, step b: rotating the tuning element 8 about a predetermined angle 12 until at least one mode or wavelength hop substantially has occurred, step c: rotating the dispersion element 10 about an arbitrary angle 16, step d: rotating back the tuning element 8 about the predetermined angle 12 of step a, and repeating steps a to d with increasing or decreasing rotating angle 16 of step c until substantially no mode or wavelength hops during rotation of the tuning element 8 are detected in step b, and using the overall rotating angle 16 of step c per rotating angle 12 of step b to evaluate an approximation of the function which determines the rotating angle 16 of the dispersion element 10 per rotating angle 12 of the tuning element 8. The approximation can be done by known approximation methods. The more repeats of steps a to d are performed the more exact is the predetermination.
The apparatus of
Similarly to the embodiment of
Additionally, it is possible to have variations in the position of the axes 14, 6a, 8a, 10a, 10b, 10c during the rotation about the axes 14 or 10a respectively the movement along axis 10c. E.g. these variations can be caused by the piezo-electric element or rotation stage for rotating the cavity end element 6, the tuning element 8 or the dispersion element 10 respectively the movement of the dispersion element 10. However, these variations can be corrected by calibrating the inventive apparatus 1 or 2.
| Patent | Priority | Assignee | Title |
| 6901094, | Dec 06 2002 | Agilent Technologies, Inc | Operating point determination for mode-selection laser |
| 7260122, | Nov 24 2004 | Wisconsin Alumni Research Foundation | Modeless wavelength-agile laser |
| 7596164, | Apr 09 2001 | Cymer, Inc. | Control system for a two chamber gas discharge laser |
| 7756171, | Oct 21 2008 | Cymer, LLC | Method and apparatus for laser control in a two chamber gas discharge laser |
| Patent | Priority | Assignee | Title |
| 5319668, | Sep 30 1992 | Newport Corporation | Tuning system for external cavity diode laser |
| 5867512, | Feb 10 1997 | Tuning arrangement for a semiconductor diode laser with an external resonator | |
| 6304586, | Sep 10 1999 | NEW FOCUS, INC | Method and apparatus for modulating a tunable laser |
| JP5267768, |
| Executed on | Assignor | Assignee | Conveyance | Frame | Reel | Doc |
| Mar 19 2002 | Agilent Technologies, Inc. | (assignment on the face of the patent) | / | |||
| May 13 2002 | AGILENT TECHNOLOGIES DEUTSCHLAND GMBH | Agilent Technologies, Inc | ASSIGNMENT OF ASSIGNORS INTEREST SEE DOCUMENT FOR DETAILS | 012936 | /0112 |
| Date | Maintenance Fee Events |
| Feb 21 2008 | M1551: Payment of Maintenance Fee, 4th Year, Large Entity. |
| May 07 2012 | REM: Maintenance Fee Reminder Mailed. |
| Sep 21 2012 | EXP: Patent Expired for Failure to Pay Maintenance Fees. |
| Date | Maintenance Schedule |
| Sep 21 2007 | 4 years fee payment window open |
| Mar 21 2008 | 6 months grace period start (w surcharge) |
| Sep 21 2008 | patent expiry (for year 4) |
| Sep 21 2010 | 2 years to revive unintentionally abandoned end. (for year 4) |
| Sep 21 2011 | 8 years fee payment window open |
| Mar 21 2012 | 6 months grace period start (w surcharge) |
| Sep 21 2012 | patent expiry (for year 8) |
| Sep 21 2014 | 2 years to revive unintentionally abandoned end. (for year 8) |
| Sep 21 2015 | 12 years fee payment window open |
| Mar 21 2016 | 6 months grace period start (w surcharge) |
| Sep 21 2016 | patent expiry (for year 12) |
| Sep 21 2018 | 2 years to revive unintentionally abandoned end. (for year 12) |